Aqueous dispersion containing pyrogenically produced metal oxide particles and phosphates

ABSTRACT

Aqueous dispersion containing pyrogenically produced oxide particles of titanium, zinc, iron or cerium having an average particle size, expressed as a median value, in the dispersion of less than 200 nm, the particle sizes of the oxide particles are not distributed symmetrically in the dispersion, the dispersion contains as dispersing agent at least one (poly)phosphate corresponding to the general formula I and has a pH of 4.5 to 7.5. It is prepared by dispersing a stream of a preliminary dispersion by means of a high-energy mill. It can be used in sunscreen formulations.

This invention relates to an aqueous dispersion containing pyrogenicallyproduced metal oxide particles and (poly)phosphates as dispersing agent,a process for preparing this dispersion and its use in the preparationof cosmetic formulations, in particular sunscreen formulations.

Cosmetic preparations containing UV filters, such as creams or lotions,which are largely transparent on the skin and pleasant to apply, areused for the protection of the skin against excessively intensive UVradiation.

They contain as UV filters one or more organic compounds which absorbUVB radiation (290 to 320 nm) and UVA radiation (320 to 400 nm) in thewavelength range between 290 and 400 nm.

The higher-energy UVB radiation causes the typical symptoms of sunburnand the suppression of the immune defence system, while the UVAradiation, which penetrates deeper into the dermal layers, causes thepremature ageing of the skin. As the combined action of the two types ofradiation is reported to favour the development of light-induced skindiseases, such as skin cancer, the search for possible ways ofsignificantly further improving the UV protection already achievedaccordingly began early on.

It is known that ultrafine pigments based on metal oxides can alsoscatter, reflect and absorb UV radiation. Accordingly, their highlydisperse formulations are an effective supplement to the organic UVfilters in sunscreens.

As is generally known, ultrafine titanium dioxide is variously used forthis purpose in cosmetic formulations, as it is both chemically inertand toxicologically safe and leads neither to skin irritation nor tosensitisation. It is the currently most frequently used and mostimportant mineral light protectant. Besides titanium dioxide, ultrafinezinc oxide is increasingly being used, a distinction being made betweencoarse material (pigment) and fine material (micropigment). In the caseof micropigments, the average primary particle size is for the most partappreciably below 200 nm, mostly within the range of 10 to 100 nm, as arule below 50 nm.

The smallest particles obtained during the preparation of the pigmentsare referred to as primary particles. Primary particles may be in theform of individual crystallites or else in the form of severalcrystallites densely intergrown across the faces.

Particles consisting of several primary particles, where the primaryparticles are intergrown across the faces, are referred to asaggregates.

An agglomerate means an amalgamation of primary particles or aggregateswhich are held together by attractive forces such as, for example,hydrogen bridge bonds.

The coarse pigment (0.2 to 0.5 μm) absorbs or reflects the radiationwidely and relatively constantly over the entire UV region and theregion of visible light, whereas the fine material shows a definiteincrease in activity in the UV region accompanied by a simultaneous lossof activity in the long-wave UVA and in particular in the visibleregion. As only a little visible light is reflected, preparations basedon this active ingredient are therefore largely transparent.

The metal oxides used in sunscreen formulations may exhibit anundesirable photoactivity which causes, for example, the formation ofreactive species such as hydroxyl radicals. Attempts have therefore beenmade to suppress the formation of these species by means of inorganicand organic surface components such as, for example, Al₂O₃. SiO₂ and/orfatty acids(salts), siloxanes or phosphates.

EP-A-154150 describes the production of metal oxide particles, thesurfaces of which are coated with alkyl phosphates, in an organicsolvent, which after the reaction has to be removed by distillation. Theparticles thus obtained are used in water-repellent sunscreenformulations.

U.S. Pat. No. 5,453,267 describes the preparation of sunscreenformulations containing 0.5 to 30 wt. % titanium dioxide having anaverage primary particle size of less than 100 nm and 0.025 to 30 wt. %phosphate anions. The titanium dioxide can be stirred into a dispersioncontaining the components of the formulation and the phosphate anions.Alternatively, phosphate-coated titanium dioxide particles, prepared byintroducing titanium dioxide into an aqueous, phosphate-containingsolution, can be used. The particles in the dispersion thus obtained arenot suitable, however, for further use in a sunscreen formulation, buthave to be ground several times.

In cosmetic, transparent formulations it is important that the particlesbe as small as possible, so that they cannot be detected on the skinjust with the naked eye. At the same time, the UV protective action of asunscreen should not be diminished and the particles should notprecipitate out during storage.

To this end, the aggregated and agglomerated metal oxide particles aredispersed. By this is meant the introduction, dispersion and uniformdistribution of solids in a liquid phase.

It is known that an increasingly fine division of the particles isaccompanied by increased problems in dispersion, so that the dispersionprocess is altogether one of the most expensive steps in the preparationof cosmetic formulations. The demands of practical operation thereforerender it expedient to separate this part from the preparation of theactual cosmetic formulations and to provide stable aqueous dispersionswhich have as high a content as possible of ultrafine metal oxideparticles and preferably are of low viscosity or are at least stillpumpable or flowable.

An important feature of a dispersion is the size of the dispersedparticles in the dispersion. This size is referred to as the secondaryparticle size and describes primary particles, aggregates andagglomerates which are present in the dispersion. In contrast to aspecification exclusively of the primary particle size, a specificationof the secondary particle size describes the actual situation in thedispersion and in the sunscreen formulation.

The dispersion of the agglomerates and wetting of the newly createdsurfaces is possible with the aid of dispersing devices such asdissolvers, ball mills, Rotor-Stator machines, the degree of dispersionbeing dependent upon the energy introduced. The energy of thesedispersing devices is insufficient for the very fine secondary particlesizes required in cosmetic and sunscreen formulations.

In EP-A-876 841 the preparation of a titanium dioxide dispersion bymeans of a high-energy mill is described, with an average particle sizeof 0.16 μm being achieved in the dispersion. The dispersion isstabilised by the addition of acetic acid. This type of stabilisation isunsuitable for cosmetic applications because, firstly, the acetic acidhas a strong self-odour and, secondly, the anticipated stability in therange of ca. pH 4.5 to 7.5 relevant for cosmetic applications is low, asthis range is close to the isoelectric point of titanium dioxide.

Accordingly, the object of the present invention is to prepare a highlyconcentrated, aqueous dispersion of ultrafine metal oxide particles,which is stable in the physiologically favourable pH range of 4.5 to 7.5and has a low viscosity and a decreased photocatalytic activity ascompared with prior art.

The object is achieved by an aqueous dispersion containing pyrogenicallyproduced oxide particles of titanium, zinc, iron or cerium having anaverage particle size, expressed as a median value, in the dispersion ofless than 200 nm, characterised in that the particle sizes of the oxideparticles are not distributed symmetrically in the dispersion and thedispersion contains as dispersing agent at least one (poly)phosphatecorresponding to the general formula I

-   -   M=H, an alkali metal, alkaline-earth metal, ammonium ion, Zn²⁺,        Al³⁺, Fe²⁺, Fe³⁺,    -   a=1 or if M is a divalent cation, a=½, if M is a trivalent        cation, a=⅓        with M being identical or different, and which has a pH value of        4.5 to 7.5.

A non-symmetrical distribution means that the arithmetic mean of thedistribution is greater than the median value. Non-symmetricaldistributions include both “oblique” monomodal and multimodaldistributions.

The term mean value signifies the arithmetic mean of the volume-weightedparticle-size distribution. The median value is the d₅₀ value of thevolume-weighted particle-size distribution.

The non-symmetrical distribution of the particle sizes in the dispersionaccording to the invention in the presence of (poly)phosphates issurprising. In the case of the dispersion of pyrogenic, aggregated metaloxide particles, one would have expected a symmetrical normaldistribution, which is identified from the fact that the ratio of meanvalue and median value of the distribution equals 1. Thus, for example,the dispersion of pyrogenically produced aluminium oxide in the presenceof (poly)phosphates by means of high dispersive energies results in asymmetrical normal distribution.

The non-symmetrical distribution of the dispersion according to theinvention means that a majority of the particles has the finenessrequired for cosmetic applications, while a smaller portion of coarserparticles has a beneficial influence on the stability and rheology ofthe dispersion.

Pyrogenically produced oxide particles of titanium, zinc, iron andcerium include both oxide particles resulting from a flame hydrolysisand oxide particles resulting from a flame oxidation. During the flamehydrolysis, precursors of the metal oxides, for example, metal halidesor organometallic compounds, undergo combustion in an oxyhydrogen flame,the precursor being hydrolysed. This synthesis, originally described forpyrogenic silicon dioxide, can also be used, for example, for theproduction of titanium dioxide. During the flame oxidation, usuallymetal vapour is oxidised to the metal oxide in an oxygen atmosphere. Theoxidation of zinc vapour to zinc oxide may be cited as an example.

From the toxicological and dermatological aspect, the safe compoundssuch as cerium oxide, zinc oxide, iron oxide and in particular titaniumdioxide are suitable for cosmetic formulations.

The oxide particles may also include mixed oxide particles, dopedparticles or coated particles of titanium, zinc, iron and cerium withone another and/or with silicon and/or aluminium. The BET surface areaof the oxide particles may vary over the wide range of 5 to 200 m²/g.

The surface of the above-mentioned metal oxide particles may also bemodified by means of organic compounds, so that hydrophilic orhydrophobic surfaces are obtained. Examples of metal oxide particlesmodified by means of organic compounds are described, for example, inDE-A-42 02 695, EP-A-1 078 957, EP-A-924 269, EP-A-722 992. Theparticles described in these documents can be used according to theinvention, but those having hydrophobic surfaces are less suitable.

The metal oxide particles used according to the invention may, forexample, be commercially available products which are obtainable underthe respective trade names, also with inorganic or organic coatings suchas, for example, micro titanium dioxide MT 100 AQ and MT 150 W(Tri-K-Tayca), UV-titanium M 212 (Kemira), and titanium dioxide P-25(Degussa) and TN-90 (Nippon Aerosil). A titanium dioxide modified byorganic compounds might be, for example, T 805 (Degussa).

Here, titanium dioxide P-25 (Degussa) having a BET surface area of ca.50 m²/g and TN 90 (Nippon Aerosil) having a BET surface area of ca. 90m²/g have proved to be particularly advantageous. The crystallographiccomposition of these oxides is about 80% anatase and about 20% rutile.They are distinguished by having high cosmetic acceptance and very goodwater resistance.

Preferably, the following (poly)phosphates corresponding to the generalformula I are used:

-   -   alkali-metal phosphates, such as monosodium phosphate, disodium        phosphate, trisodium phosphate, monopotassium phosphate,        dipotassium phosphate and tripotassium phosphate; also        polyphosphates, such as pentasodium triphosphate, Graham's salt,        Maddrell's salt, Kurrol's salt, pyrophosphates such as        tetrasodium pyrophosphate.

The (poly)phosphates may also be used in the form of mixtures. Mixturesof corresponding (poly)phosphates, which form a buffer in the pH rangebetween 4 and 8, or more commonly between 5 and 7, are particularlypreferred. Thus, for example, a 4:1 mixture of monosodium phosphate:disodium phosphate produces a buffer at about pH 6.

The dispersion may preferably contain 20 to 60 wt. % metal oxideparticles. The range is particularly preferably from 30 to 50 wt. %.

The dispersion may preferably contain 0.2 to 30 wt. % (poly)phosphatescorresponding to formula I. The range is particularly preferably from0.5 to 15 wt. %.

In addition to the above-mentioned components, the dispersion maycontain acids, preferably acids such as phosphoric acid, sulfuric acid,hydrochloric acid, nitric acid or carboxylic acids, for the regulationof the pH.

The dispersion may also contain known auxiliary substances andadditives, such as ethanol, propanol, butanol, propylene glycol,butylene glycol, pentylene glycol, hexylene glycol, alkoxylates, glycolethers, glycols, polyethylene glycols, polypropylene glycols,polybutylene glycols, glycerol ester ethoxylate, glycerol, polyglycerol,sorbitol, sucrose, fructose, galactose, mannose, polysorbates, starches,xanthan gum, carrageenan gum, cellulose derivatives, alginates, glycolesters, sorbitan esters, opacifiers, solubilisers, ethoxylated fattyalcohols, sodium chloride, sodium sulfate, magnesium sulfate, buffersystems, cholesterol, pantothenic acid, ascorbic acid, polyacrylicacids, carbomers.

Within the pH range of 4.5 to 7.5, the dispersion according to theinvention may exhibit a zeta potential of less than −20 mV. At values ofless than −20 mV, the dispersion has a particularly high stability. Thezeta potential is the outwardly active potential of the particles and isa measure of the electrostatic interaction between individual particles.It is a factor in the stabilisation of suspensions and in particular ofdispersions containing dispersed, ultrafine particles. At a zetapotential value of <−20 mV or >+20 mV there is a strong repulsionbetween the particles and the dispersions remain stable. At valueswithin this range, the repulsion is so slight that the van de Waalsforces permit the formation of agglomerates and this leads to theundesirable sedimentation of the particles.

The dispersion according to the invention may also have a viscosity ofless than 2000 mPas at a shear rate of 100 s-1.

The invention also provides a process for preparing the dispersionaccording to the invention, which is characterised in that a stream ofan initial dispersion, which contains pyrogenically produced metal oxideparticles, at least one (poly)phosphate corresponding to the generalformula I, water and optionally additional auxiliary substances, isdivided into at least two substreams, these substreams are placed in ahigh-energy mill under a pressure of at least 500 bar, preferably 500 to1500 bar, particularly preferably 2000 to 3000 bar, are released througha nozzle and impact upon one another in a gas- or liquid-filled reactionchamber and are ground.

High-energy mills are commercially available devices. For example, anUltimaizer from Sugino or the device described in DE-A-100 37 301 aresuitable for preparing the dispersion according to the invention.

The initial dispersion can be prepared, for example, by means ofdissolvers, Rotor-Stator machines or ball mills. Preferably,Rotor-Stator machines are used.

The stream of dispersion can be recirculated, so that the dispersion isground several times by means of a high-energy mill.

The dispersions according to the invention are used preferably in thepreparation of cosmetic formulations, such as make-up, coloured powders,lipsticks, hair colorants, day creams and in particular sunscreenpreparations and can be supplied in the conventional forms such as, forexample, W/O- or O/W-dispersions (emulsions), gels, creams, lotions,sprays.

The dispersions obtained are distinguished by an extremely fine divisionof the dispersed solid matter, a long-term stability in storage, a lowviscosity and a high photostability.

EXAMPLES

The constituents of the dispersion according to Table 1, except for theTiO₂ particles, were placed in a receiver (batch size approx. 75 kg).The particles were then drawn in through the suction valve of an YstralConti-TDS 3 under shearing conditions and, when drawing-in was complete,shearing was continued at 3000 rev/min for 15 minutes (Sample 0, seeTable 2). This preliminary dispersion was passed three times through thehigh-energy mill Sugino Ultimaizer HJP-25050 at a pressure of 2500 barand with diamond nozzles of 0.3 mm in diameter and a sample was removedafter each passage (first passage is sample 1, second passage is sample2, and so on; see Table 2). Viscosities are shown in Table 3.

The dispersions according to the invention are stable in storage at roomtemperature for more than 6 months and at 50° C. for more than 1 month.TABLE 1 Formulations [in wt. %] Example 1 2 3 TiO₂ ⁽¹⁾ 40 40 33 sodiumpolyphosphate⁽²⁾ 0.6 — 0.7 pentasodium triphosphate — 0.6 — deionisedwater 59.4 59.4 66.3 pH value 6.52 7.25 6.79 zeta potential [mV/pH]⁽³⁾−29.2/5.0 −30.5/5.2 −27.3/5.2⁽¹⁾Ex. 1, 2: P25(Degussa AG); Ex. 3: TN90 (Nippon Aerosil);⁽²⁾Graham's salt⁽³⁾determined by means of a DT-1200 instrument from DispersionTechnology Inc., USA;

TABLE 2 Particle-size distribution [in nm]¹⁾ Example Sample 1 2 3 0median 261 242 221 mean 287 285 245 mean/median 1.10 1.18 1.11 1 median254 195 150 mean 265 239 191 mean/median 1.04 1.23 1.27 2 median 107mean n.d. n.d. 142 mean/median 1.33 3 median 157 102 96 mean 197 156 131mean/median 1.25 1.53 1.36¹⁾determined by means of a Malvern Zetasizer 3000 HSa using theprinciple of dynamic light scattering. The result of the volume-weightedcontinuous analysis is shown.

TABLE 3 Viscosity⁽¹⁾ of the dispersions [in mPas] relative to the shearrate [in s⁻¹] Example Sample Shear rate 1 2 3 0 1/100 10500/15027500/290 29200/335 2 1/100  6900/125  9850/265 30800/425⁽¹⁾viscosity was determined as viscosity flow by means of a viscosimeterPhysica MCR 300 and the CC27 measuring system

1. Aqueous dispersion containing pyrogenically produced oxide particlesof titanium, zinc, iron or cerium having an average particle size,expressed as a median value, in the dispersion of less than 200 nm,wherein the particle sizes of the oxide particles are not distributedsymmetrically in the dispersion and the dispersion contains asdispersing agent at least one (poly)phosphate corresponding to thegeneral formula I

M=H, an alkali metal, alkaline-earth metal, ammonium ion, Zn²⁺, Al³⁺,Fe²⁺, Fe³⁺, a=1 or if M is a divalent cation, a=½, if M is a trivalentcation, a=⅓ with M being identical or different, and wherein saidaqueous dispersion has a pH value of 4.5 to 7.5.
 2. Aqueous dispersionaccording to claim 1, wherein the metal oxide particles include theoxides of titanium, zinc, iron, cerium, mixed oxides thereof, and themixed oxides of the above-mentioned oxides with aluminium or silicon. 3.Aqueous dispersion according to claim 1, wherein the surface of themetal oxide particles is modified by means of organic compounds. 4.Aqueous dispersion according to claim 1, wherein said aqueous dispersioncontains 20 to 60 wt. %, metal oxide particles.
 5. Aqueous dispersionaccording to claim 1, wherein said aqueous dispersion contains 0.2 to 30wt. % of (poly)phosphates corresponding to the general formula I. 6.Aqueous dispersion according to claim 1, wherein said aqueous dispersioncontains other auxiliary substances and additives.
 7. Aqueous dispersionaccording to claim 1, wherein within the pH range of 4.5 to 7.5 saidaqueous dispersion exhibits a zeta potential of less than −20 mV. 8.Aqueous dispersion according to claim 1, wherein said aqueous dispersionhas a viscosity of less than 2000 mPas at a shear rate of 100 s-1. 9.Process for preparing the dispersion according to claim 1, wherein astream of an initial dispersion, which contains pyrogenically producedmetal oxide particles, in each case at least one (poly)phosphatecorresponding to the general formula I, water and optionally additionalauxiliary substances, is divided into at least two substreams, thesesubstreams are placed in a high-energy mill under a pressure of at least500 bar and are released through a nozzle and impact upon one another ina gas- or liquid-filled reaction chamber and are ground.
 10. Processaccording to claim 9, wherein the dispersion is ground several times bymeans of a high-energy mill.
 11. A method for preparing a cosmeticformulation comprising adding an aqueous dispersion according to claim 1to said cosmetic formulation.
 12. Aqueous dispersion according to claim1, wherein said aqueous dispersion contains 30 to 50 wt. % metal oxideparticles.
 13. Aqueous dispersion according to claim 1, wherein saidaqueous dispersion contains 0.5 to 15 wt. % of (poly)phosphatescorresponding to the general formula I.
 14. Process for preparing thedispersion according to claim 1, wherein a stream of an initialdispersion, which contains pyrogenically produced metal oxide particles,in each case at least one (poly)phosphate corresponding to the generalformula I, water and optionally additional auxiliary substances, isdivided into at least two substreams, these substreams are placed in ahigh-energy mill under a pressure of 500 to 1500 bar and are releasedthrough a nozzle and impact upon one another in a gas- or liquid-filledreaction chamber and are ground.
 15. Process for preparing thedispersion according to claim 1, wherein a stream of an initialdispersion, which contains pyrogenically produced metal oxide particles,in each case at least one (poly)phosphate corresponding to the generalformula I, water and optionally additional auxiliary substances, isdivided into at least two substreams, these substreams are placed in ahigh-energy mill under a pressure of 2000 to 3000 bar and are releasedthrough a nozzle and impact upon one another in a gas- or liquid-filledreaction chamber and are ground.